The invention relates to water-soluble, cross-linkable copolymers, processes for their preparation and their use.
Water-soluble polymers with ionic groups, so-called polyelectrolytes, have been known for a long time. These substances are polymers in which ionic groups, such as, for example, carboxylate or sulphonate groups, are firmly bonded to the polymer backbone via a chemical bond. As a rule good water-solubility and quite specific rheological properties, in general a high viscosity in an aqueous medium, result from this.
These polyelectrolytes are prepared by free radical polymerization of water-soluble, ethylenically unsaturated compounds which contain ionic groups. Examples of these are: acrylic acid, N-methylolacrylamide, acrylamido-2-methylpropanesulphonic acid (AMPS) and styrenesulphonic acid. Very high molecular weight polymers, aqueous solutions of which have a high viscosity, result from this preparation.
EP-B 94898 (U.S. Pat. No. 4,736,005) describes the preparation of very high molecular weight (molecular weight &gt;1,000,000) terpolymers of 30 to 95% of dimethylacrylamide (DMA), 0.1 to 10% of N-methylolacrylamide (NMA) and 4 to 50% of acrylamidomethylpropanesulphonate (AMPS) and their use as rheological additives in the production of crude oil.
DE-A 2618898 (U.S. Pat. No. 3,965,032) relates to colloidal dispersions which comprise a water-soluble or water-dispersable copolymer polyelectrolyte with nonionic hydrophobic units, for example methyl methacrylate, and ionic hydrophilic units, for example AMPS, the preparation of which is carried out in solution polymerization in a water-miscible solvent. In concrete terms, copolymers which comprise the hydrophobic component in an excess of 2:1 to 6:1 are described.
EP-A 629650 (U.S. Pat. No. 5,278,222) and EP-A 671435 describe water-soluble and water-insoluble polymers of hydrophobic monomer units and 15 to 80% by weight or 30 to 50% by weight of monomers containing sulphonate groups as spraying auxiliaries in the spray drying of aqueous polymer dispersions.
Water-soluble polymers with high contents of crosslinkable water-soluble monomer units, such as N-methylolacrylamide (NMA), would in principle be of interest in the form of aqueous solutions thereof as binders for coating compositions or adhesives. Another conceivable use would be that as dispersing agents in aqueous polymer dispersions. Water-soluble polymers with a high NMA content have the tendency to have high molecular weights, with a correspondingly high viscosity of the aqueous solution. Water-soluble acrylic compounds, such as acrylic acid or N-methylolacrylamide, which is customary as a crosslinking agent, in fact tend to have very high degrees of polymerization, which severely impedes widespread use because of the resulting high viscosities.
Another disadvantage of polyelectrolytes based on water-soluble monomers is that such polyelectrolytes are often incompatible with a polymer dispersion and the aqueous polyelectrolyte solution separates off from the polymer dispersion as a serum.